DOM Manipulation (append/prepend/before/after) – This code has also undergone a large rewrite along with some of the logic for executing inline script elements.

.offset() – Another method that has been completely rewritten.

Event Namespaces – The logic for handling namespaced events has been completely rewritten.

Event Triggering – When triggering an event the event now bubbles up the DOM – this is likely to cause some problems.

While we wonâ€™t get into the particulars of all the new features that are in jQuery 1.3 (weâ€™ll do that later, when itâ€™s ready for final release – scheduled for January 14th) we do appreciate any/all feedback that you can provide.

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* What is Sizzle?

From the link above readme:

A pure-JavaScript CSS selector engine designed to be easily dropped in to a host